The invention relates generally to systems, devices and methods for the compression and/or expansion of a gas, such as air, and particularly to a system, device and method for optimizing the energy efficiency of a compressed air energy storage system.
Traditionally, electric power plants have been sized to accommodate peak power demand. Moreover, electric power plant sizing must take into account their maximum power output, minimum power output, and a middle power output range within which they most efficiently convert fuel into electricity. Electric power plants are also constrained in terms of how quickly they can start-up and shut-down, and it is commonly infeasible to completely shut-down a power plant. The combination of power output constraints, and start-up and shut-down constraints, restricts a power plant's ability to optimally meet a fluctuating power demand. These restrictions may lead to increased green house gas emissions, increased overall fuel consumption, and/or to potentially higher operating costs, among other drawbacks. Augmenting a power plant with an energy storage system may create an ability to store power for later use, which may allow a power plant to fulfill fluctuating consumer demand in a fashion that minimizes these drawbacks.
An energy storage system may improve overall operating costs, reliability, and/or emissions profiles for electric power plants. Existing energy storage technologies, however, have drawbacks. By way of example, batteries, flywheels, capacitors and fuel cells may provide fast response times and may be helpful to compensate for temporary blackouts, but have limited energy storage capabilities and may be costly to implement. Installations of other larger capacity systems, such as pumped hydro systems, require particular geological formations that might not be available at all locations.
Intermittent electric power production sites, such as some wind farms, may have capacities that exceed transmission capabilities. Absent suitable energy storage systems, such intermittent power production sites may not be capable of operating at full capacity. The applicants have appreciated that intermittent production sites may benefit from a storage system that may be sized to store energy, when the production site is capable of producing energy at rates higher than may be transmitted. The energy that is stored may be released through the transmission lines when power produced by the intermittent site is lower than transmission line capacity.
Electric power consumption sites, such as buildings, towns, cities, commercial facilities, military facilities, may have consumption that periodically exceeds electricity transmission capabilities. Absent suitable energy storage systems, such power consumers may not be capable of operating at preferred levels. The applicants have appreciated that transmission constrained consumption sites may benefit from a storage system that may be sized to store energy, when the consumption site is consuming energy at rates lower than may be transmitted, and that the transmitted energy that is not immediately consumed may be stored. The energy that is stored may be released to the consumers when power consumption of the consumers is higher than the transmission line capacity. The energy may also be stored during off-peak time periods (e.g., at night) when electricity prices are generally less expensive and released during peak time periods (e.g., during the day) when electricity prices are generally more expensive.
Compressed air energy storage systems (CAES) are another known type of system in limited use for storing energy in the form of compressed air. CAES systems may be used to store energy, in the form of compressed air, when electricity demand is low, typically during the night, and then to release the energy when demand is high, typically during the day. Such systems include a compressor that operates, often at a constant speed, to compress air for storage. Turbines, separate from the compressor, are typically used to expand compressed air to produce electricity. Turbines, however, often require the compressed air to be provided at a relatively constant pressure, such as around 35 atmospheres. Additionally or alternatively, air at pressures higher than 35 atmospheres may need to be throttled prior to expansion in the turbine, causing losses that reduce the efficiency of the system, and/or reduce the energy density that a storage structure may accommodate. Additionally, to increase electrical energy produced per unit of air expanded through the turbine, compressed air in such systems is often pre-heated to elevated temperatures (e.g., 1000° C.) prior to expansion by burning fossil fuels that both increases the cost of energy from the system and produces emissions associated with the storage of energy.
Known CAES-type systems for storing energy as compressed air have a multi-stage compressor that may include intercoolers that cool air between stages of compression and/or after-coolers that cool air after compression. In such a system, however, the air may still achieve substantial temperatures during each stage of compression, prior to being cooled, which will introduce inefficiencies in the system. Thus, there is a need to provide for CAES type systems that have improved efficiencies.
A CAES system may be implemented using a hydraulic drive system comprised of hydraulic components including components such as hydraulic pumps used to drive working pistons. Therefore, there is also a need for systems and methods to obtain a high efficiency output of a compressed air energy storage system, or other systems used to compress and/or expand gas, including controls and operating modes that leverage bi-directional piston movement during operation of such a system.